Outgasses from construction materials

Michael Gall and Helmut Bauer of M+W Zander, Germany, highlight how the determination of compounds outgassed from construction materials can help determine which products are most suitable for controlled environments

Figure 1: Samples of construction materials

Michael Gall and Helmut Bauer of M+W Zander, Germany, highlight how the determination of compounds outgassed from construction materials can help determine which products are most suitable for controlled environments.

Chemical contamination is one of the key topics in chip production at present. With a growing demand for semiconductor devices, organic contaminants adhering to silicon wafer surfaces and the lenses used in lithographic processes have become a serious problem for chip manufacturers.

The outgassing of building and construction materials contributes substantially to the contamination of cleanroom environments. That is why outgassing tests for cleanroom construction materials and products are almost as important as the evaluation of organic contaminants in the cleanroom air itself.

Producers and suppliers of various components worldwide are obliged to investigate their materials in several ways before releasing them for cleanroom use.

The globally active M+W Zander group has set up a laboratory in its Technology Centre that enables up-to-date analysis of outgassing parameters. The company also has considerable experience in evaluating the significance of gaseous concentrations for the requested cleanroom application, and has established an extensive database for certified products and materials.

Whether it is intended as a floor covering, adhesive, sealing compound or filter compound, each component is examined for its intended purpose and the results must fall within specified limits. Depending on the future application, multiple tests may be required to determine the varying levels of outgassing. The result of these investigations is a detailed description of the materials that should be used for a cleanroom configuration.

The outgassing procedures used comply with the latest recommendations of the International Technology Roadmap for Semiconductors (ITRS).

The investigation procedure answers the following questions:

• Why is it necessary to test cleanroom production materials?

• What is needed to carry out the investigation?

• What is to be tested?

• What sample preparation is necessary?

Finally, we have to decide which of the parameters are of interest and what are the critical amounts of organic and inorganic outgassing contamination.

The desired result of the investigation is a list of materials and samples that are specified for use in cleanroom components.

Why test?

We need contamination control for organic compounds because some of the compounds critical for the manufacturing processes adhere to silicon wafer surfaces or lenses. Examples of critical substances are: organophosphates, boron, condensable organics and ammonia. One of the most important points in this connection is the surface area of the intended application within the cleanroom. According to whether a material is used extensively or selectively, there are different limiting values for release. First of all, we differentiate between “cleanroom materials” and “cleanroom products”.

What can be tested?

Any material that can be broken down into a small sample size can be tested, irrespective of the later application. By following a precisely defined procedure, the results can be compared easily. More complicated components, such as filters that are made of many different materials, can be examined as one. In this case, outgassing is measured in one step, based on the proportion of the materials that come into contact with air.

In general, there are two main procedures for outgassing detection, depending on the intended application. The first procedure detects direct material outgassing through thermal extraction and measurement of the gaseous phase over the material sample. The setup has an exposure temperature of 90°C at ambient pressure. Under these conditions the ratio of outgassing compounds increases to a range that can be measured easily without decomposing the substance. This derives from the fact that the amount of outgassing at 90°C is more than 100 times that at 20°C.

The screening technique is by dynamic headspace gas chromatography mass spectrometry (GC-MS). The result is a table of the 10 most abundant compounds. It includes the sum of organic outgassing in a range of boiling points above 150°C as well as compounds with boiling points above 300°C. The search for amines and refractories, such as organo-phosphorous compounds, siloxanes and other substance classes, is evaluated closely against the latest semiconductor roadmaps.

The next outgassing test measures the contamination level quantitatively. The quantity of outgassing (contamination level) experienced in a cleanroom is a function of multiple parameters. Building an environment that is a perfect replica of the intended operational area would be costly. So a testing environment is built that includes a whole set of criteria of acceptance.

M+W Zander has worked with and within cleanrooms over many years and knows precisely which of the external and internal factors are of most importance. Based on this extensive experience, the outgassing values lead to a recommendation for the level of their subsequent usage in cleanroom construction.

Sample preparation

The samples to be tested need to achieve certain requirements. They are prepared from the original coating materials, in accordance with their future field of application. The required size is 30 x 10mm (length x width) with a thickness of approximately 10mm and the layer composition (thickness of the layers and their sequence) of multilayer coatings has to be in accordance with the planned use of the coating. Substrate materials are stainless-steel, aluminum foil or other materials that are resistant to temperature changes at 100°C and do not contribute to organic outgassing. Of great importance is a detailed description of the sample, which must include the sample’s fabrication date and the weights of the applied materials per sample or per unit of area, e.g. per cm2.

Each sample needs to be accompanied by detailed documentation, including a technical product datasheet and the material’s safety data. Finally, in the case of reactive hardening coatings, such as epoxy lacquer, it is necessary to vent the samples for 30 days after preparation and prior to testing.

This venting time is necessary to obtain representative results and is therefore performed under controlled conditions in the M+W Zander laboratory. The sampling conditions are defined according to the base-line concentration specification.

The second procedure is developed and applied for products such as particle or chemical filters. This test series is carried out at room temperature and with a nominal airflow rate. The result is the out-gassing amount at operational conditions, including each individual component that is used for the construction of the product. The screening technique is again performed using dynamic headspace gas chromatography mass spectrometry (GC-MS), but in addition ion chromatography (IC) and inductively coupled plasma mass spectrometry (ICP/MS) are used.

The final outcome is a table that summarises information about the same compounds as those detected in the material out-gassing procedure, but also including inorganic com-pounds (anions and cations), boron and iodine.

This outgassing test measures the contamination level quantitatively after a specific venting time. This value corresponds to the actual outgassing of the complete product and enables its contribution to the airborne concentration in the cleanroom to be calculated.

The laboratory needs a representative and clean specimen. Due to variations in specifications, a detailed definition of the subsequent use of each separate component is also needed. For instance, they may be used for coating, adhesive, floor tiles, filters, etc. The next important factor is time: The analysis needs 30 days of ambient air exposure for materials and five days of venting time for product components. The complete investigation includes measurement, reporting and certification for cleanroom usage.

The detection sensitivity is entirely sufficient to meet even the most ambitious specification. An order of priority for outgassing parameters may be defined and “bad candidates” may be separated easily. This clear predication makes the test procedures an ideal tool for screening of components. Certification by M+W Zander guarantees that your product can be used for cleanroom application.